The quantitative determination of optical properties of tissue is important in the areas of diagnostics and imaging. A new approach based on frequency-domain methods was developed during the previous grant period. The new method uses several high frequency modulated light sources at different distances from a light detector. Using this instrument, it is possible to achieve complete separation of the scattering from the absorption contribution to light extinction as it propagates throughout tissues. An instrument has been built that can accurately recover the entire spectrum of an absorbing substance in strongly scattering media. A particular implementation of the method has been used to continuously monitor oxygen saturation in tissues, noninvasively. The major new achievement is that non-invasive, quantitative tissue spectroscopy can be obtained on the basis of a physical model for light transport in tissues. However, the present method only applies to homogenous tissues. The aim of this proposal is to systematically approach the problem of detection and quantitation of tissue inhomogeneity. New experimental methods and theoretical approaches will be explored to construct a volume map of the absorption and scattering coefficient of thick tissues. The long term goal of this proposal is to develop the physical basis of new non-invasive diagnostic methods using light and to construct prototype instruments capable of mapping the optical parameters of tissues with sub-centimeter resolution.